1. Field of the Invention
The present invention relates to a hopper for supplying raw material to a molding machine and more particularly to a hopper drier for drying granular material of synthetic resin or the like to supply it to a molding machine.
Raw material of synthetic resin used for molding products of synthetic resin such as plastic or the like is generally supplied in the form of pellets. As is well known, when pellets contain moisture more than a predetermined level, hydrolyzation takes place during molding operation, resulting in the synthetic resin being deteriorated. Thus, molded products have reduced strength and ductility. Further, as moisture in pellets increases, molten resin becomes fluid. This results in that molten resin is excessively filled in a molding die and thereby burs are formed round molded products, causing them to have an incorrect geometrical configuration, reduced accuracy and a spoilt surface appearance. For this reason there is a necessity for sufficiently drying pellets prior to using the latter. To meet this requirement it has been proposed that pellets be dried by using atmospheric air. However, it was found that this proposal was unacceptable for the reason mentioned below.
As is known, atmospheric air contains certain moisture. If it is assumed that outside air has a temperature of 20.degree. C., an absolute pressure of 760 mm Hg and a relative humidity of 75%, the moisture contained in air has a unit volume of 1 m.sup.3 amounts to 10.251 grams. If this wet air is introduced into a molding machine by operating a blower having a capacity of 2.5 m.sup.3 per minute, the result is that moisture passes through pellets at a rate of 2630.25 grams per hour. In a case where nylon resin having a high moisture absorptivity is employed for a molding operation, moisture more than that removed by drying is given to the nylon pellets.
2. Description of the Prior Art
In view of these facts, a molding factory where drying is required at a high rate has hitherto been equipped with a hopper having a drier incorporated therein for drying pellets prior to delivering them to a molding machine.
To facilitate understanding of the present invention it will be helpful for a typical conventional hopper drier to be described below with reference to FIG. 1. As illustrated in the drawing, the conventional hopper drier includes a hopper housing 11 in which pellets to be supplied to a molding machine are stored, a dehumidified air preparing unit 12 for preparing dehumidified air to be delivered to the hopper housing 11 and a heating column 13 in which dehumidified air is heated up to an elevated temperature. Specifically, the dehumidified air preparing unit 12 includes two dehumidifying columns 14 and 15 with dehumidifying material filled therein, a heater 16 for drying dehumidifying material in the dehumidifying columns 14 and 15 with the aid of hot air having a temperature higher than 300.degree. C. to regenerate it, a cooling apparatus 17 for cooling dehumidifying material which has been regenerated by hot air and three-way valves 18 for delivering only to the one dehumidifying column air coming from the heater 17 and the cooling apparatus 17 and delivering to the heating column 13 air which has passed through the other dehumidifying column. The heating column 13 is provided with a heater 19 so that dry air introduced into the heating column 13 from the dehumidifying column 14 via the three-way valve 18, the blower 20, the filter 21 and the stop valve 22 is heated up to an elevated temperature and thereafter it is delivered to the hopper drier housing 11 to dry pellets stored in the latter. After passing through the hopper drier housing 11, waste air is discharged into the outside environment via a discharging port 23. As illustrated in the drawing, a part of the waste air is introduced into dehumidifying column 14 or 15 after it is cooled down in a cooling apparatus 24 and then it flows through a filter 25.
Next, when the three-way valve 18 is shifted, the dehumidifying column 15 initiates a so-called regenerating cycle in which moisture absorbed in dehumidifying material is removed. Namely, dry air taken from the dehumidifying column 14 by operating the blower 20 is caused to flow through the stop valve 26 and the cooling apparatus 17 which is now kept inoperative and it is then heated in the heater 16. Thus heated air is delivered to the dehumidifying column 15 via the three-way valve 18 and thereby dehumidifying material with moisture absorbed therein is dried. After completion of dehumidification the heater 16 is turned off and the cooling apparatus 17 starts its operation to cool dehumidifying material by allowing cooled dry air to flow therethrough. This is because of the necessity for cooling down dehumidifying material to a lower temperature close to room temperature due to the fact that dehumidification fails to be achieved satisfactorily as long as dehumidifying material is kept hot. After dehumidifying material is cooled down, the three-way valve 18 is shifted so that dry air which has passed through the dehumidifying column 15 is introduced into the heating column 13. It should of course be understood that if air cooled in the cooling apparatus 24 is to be utilized, it is caused to flow into the dehumidifying column 15 via the filter 25. At this moment the dehumidifying column 14 initiates the same regenerating cycle as mentioned above with respect to the dehumidifying column 15.
To cool dehumidifying material in the dehumidifying columns 14 and 15 it usually takes a time longer than two hours, although this varies in dependence on the amount of dehumidifying material to be cooled. Further, in the illustrated case, another drawback is that a large amount of electric power is consumed because the heater 16 requires 6 KW, the cooling apparatus 3 KW and the heater 19 in the heating column 13 3 KW. As described above, in a case where waste air to be discharged through the discharging port 23 is reused, there is a necessity for installing the cooling apparatus 23 to cool hot waste air in order to assure that dehumidifying material functions properly. Thus, a large amount of thermal energy is consumed in vain. Moreover, mechanical driving energy is required for operating the cooling apparatus 23.